I like it for what it is, but for anybody looking to use a bone devil with a little more meat on its... well, you get the idea. Anyway, I've thrown together a rough interpretation of a more 3e-ish osyluth. I'm adding it here since some have been wondering what happened to the bone devil's abilities.
[h=2]OSYLUTH (BONE DEVIL)[/h]Osyluths are bone devils gifted with additional strength and power. They act as the enforcers and inquisitors of the nine hells. Like other bone devils, the osyluths have a deadly, poisonous stinger that they can use with surprising accuracy. They are exceptionally cunning and possess natural spellcasting ability, giving them an edge during battle not afforded to their lesser kin.
Large fiend (devil), lawful evil
Armor Class 19 (natural armor)
Hit Points 157 (15d10 + 75)
Speed 40 ft., fly 40 ft.
Str 20 (+5) Dex 20 (+5) Con 20 (+5)
Int 16 (+3) Wis 15 (+2) Cha 18 (+4)
Saving Throws: Int +7, Wis +6, Cha +8
Skills: Arcana +7, Deception +8, Insight +6, Stealth +9
Damage Resistance: acid, cold, bludgeoning, piercing, and slashing from nonmagical weapons that aren’t silvered
Damage Immunities: fire, poison
Condition Immunities: poisoned
Senses Darkvision 120 ft., passive Perception 12
Languages: Infernal, telepathy 120 ft.
Challenge: 11 (??? xp)
TRAITS
Devil’s Sight: Magical darkness doesn’t impede the osyluth’s darkvision.
Fear Aura: A creature that starts its turn within 10 feet of the osyluth must make a DC 16 Wisdom saving throw against fear.
__Failed Save: The target is frightened for 1 minute. While frightened, the target must use its movement to move away from the osyluth. As an action, the frightened target can make a DC 16 Wisdom check to end this effect.
Magic Resistance: The osyluth has advantage on saving throws against spells and other magical effects.
Spellcasting: The osyluth is a 10th‐level spellcaster that uses Charisma as its magic ability (spell save DC 16). The osyluth knows the following spells:
__Cantrips – minor illusion, ray of frost, read magic
__1st level (4/day) – charm person, detect magic, sleep
__2nd level (3/day) – hold person, invisibility, mirror image
__3rd level (3/day) – dispel magic, fly, haste
__4th level (3/day) – dimension door, polymorph
__5th level (2/day) – dominate person, true seeing
ACTIONS
Multiattack: The osyluth makes three attacks: two with its claws and one with its sting.
Claw – Melee Weapon Attack: +9 to hit, reach 10 ft., one target.
__Hit: 9 (1d8 + 5) slashing damage.
Sting – Melee Weapon Attack: +9 to hit, reach 10 ft., one target.
__Hit: 14 (2d8 + 5) piercing damage plus 17 (5d6) poison damage, and the target must succeed on a DC 16 Constitution saving throw or become poisoned for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.